With the development of efficient light emitting diodes (LEDs) that emit blue or ultraviolet (UV) light, it has become feasible to produce LEDs that generate white light through phosphor conversion of a portion of the primary emission of the LED to longer wavelengths. Conversion of primary emission of the LED to secondary emissions with longer wavelengths is commonly referred to as down-conversion of the primary emission. As used herein, “primary light” or “primary emission” refers to light emitted by a light emitting diode and “secondary light” or “secondary emission” refers to light emitted by a phosphor. The unconverted primary light combines with the secondary light to produce white light.
Currently, state-of-the-art phosphor-converted LEDs are produced by mixing a phosphor in a binding medium, such as epoxy, silicone, or other similar material, which are used to encapsulate the LED. The phosphor is generally in the form of a powder that is mixed into the binding medium prior to curing. The uncured slurry containing the phosphor powder is deposited over the LED die and cured.
FIG. 1 illustrates an example of a conventional phosphor converted LED 10. The phosphor converted LED 10 includes an LED die 12 that is disposed within a reflective cup 14. The LED die 12 is encapsulated with the binding medium 16, in which phosphor particles 18 have been mixed. The binding medium 16 and phosphor particle 18 mixture will be sometimes referred to as the phosphor/epoxy mixture 19. Once the phosphor/epoxy mixture 19 is deposited over the LED die 12, the mixture is cured.
The refractive index of the mixture 19 controls the light outcoupling from the die 12 to the phosphor particles 18, as well as the light extraction from the phosphor particles 18. The refractive index of the phosphor/epoxy mixture 19 typically is only about 1.5. Moreover, the binding medium 16 conventionally used is organic and sensitive to high light flux and elevated temperatures.
The phosphor particles 18 generally are randomly oriented and interspersed throughout the binding medium 16. In operation, a portion of the primary light emitted from the active region 13 of the LED die 12 passes through the phosphor/epoxy mixture 19 without impinging on the phosphor particles 18, and is emitted by the LED lamp 10. Another portion of the primary light impinges on the phosphor particles 18, which converts the light to longer wavelengths, i.e., the phosphor particles 18 absorbs the primary light and produces a secondary emission of light with longer wavelengths. The secondary light is emitted by the LED 10 along with the unconverted primary light and slightly modified primary light (by absorption in the phosphor). The resulting correlated color temperature (CCT) of the light is thus, a function of the wavelengths of the primary light, the wavelengths of the secondary light, and the conversion efficiency of the phosphor/epoxy mixture 19, i.e., the percentage of the primary light that is converted into secondary light and is emitted by the LED 10.
A disadvantage of using a phosphor/epoxy mixture 19 is that the uniformity of the CCT in the light emitted by the LED lamp 10 is difficult to obtain. One reason for the lack of uniformity is caused by the travel distance of the emitted light through the phosphor/epoxy mixture 19. For example, as illustrated in FIG. 1, primary light emitted from the active region 13 of LED die 12 along the path illustrated by arrow 20 has a relatively short path through the phosphor/epoxy mixture 19 compared to the primary light emitted along the path illustrated by arrow 22. Accordingly, light produced by LED 10 along path 20 will have more primary light than light produced along path 22. Thus, the color of the light emitted by the LED 10 will vary depending on the viewing angle.
Moreover, conventional methods of producing phosphor-converted LEDs result in a wide variation in the CCT from one LED lamp to the next. As discussed above, the resulting CCT is dependent on factors such as the wavelengths of the primary light and the conversion efficiency of the phosphor/epoxy mixture 19. Typically, there is a small variation in the wavelengths of the light emitted from one LED die 12 to the next. Moreover, the conversion efficiency of the phosphor/epoxy mixture 19 typically varies from one device to the next. The conversion efficiency is difficult to control as it is the result of such things as non-uniformity in the sizes of the phosphor particles 18 and the settling of the phosphor particles 18 within the binding medium 16. Thus, a mixture of phosphor particles 18 and binding medium 16 that results in a desired CCT for one LED die 12 may not produce the same CCT with another LED.
Accordingly, in practice, to obtain a phosphor-converted LED lamp with a desired CCT, a number of phosphor-converted LED lamps must be produced. The LED lamps are tested to determine which, if any, produce light with the desired CCT. The LED lamps that fail to produce the desired CCT are discarded or used for other purposes.